Drying oven



March 5, 1946 DRYING OVEN Filed Ma BALD 2,395,834

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INVENTOR ATTORNEYS w. BALD 2,395,834

DRYING OVEN l0 Sheets-Sheet 3 INVENTOR BY Mia ATTORNEYS Filed March 31 1942 March 5, 1946.

- March 5,1946. w. L 2,395,834 I DRYING OVEN Fil ed March 51. 1942 10 Sheets-Sheet 4 INVENTOR ATTOR'NEYS March 5, 1946. w. BALD 2,395,834 DRYING OVEN Filed March 51 1942 10 Sheets-Sheet 5 H 10+ JET- l3! 1,37 INVENTOR ay I B wazmfi iamf ATTORNEYS March 5, 1946. w. BALD DRYING OVEN Filed March 31 1942 10 Sheets-Sheet 6 INVENTOR mad ax w ATTORNEYS W. BALD DRY'ING OVEN March 5, 1946.

Filed March 31 1942 10 Sheets-Sheet '7 A; Jai

ATTORNEYS March 5,1946. w, BALD 2,395,834

DRYING OVEN Filed March 31, 1942 I 10 Sheets-Sheet 9 l I I N f I :8 QM

March 5, 1946. v w, BALD 2,395,834

DRYING OVEN Filed March 31, 1942 10 Sheets-Sheet 10 IN VEN TOR. 2 4/ ATTOENEYS 1 Patented Mar. 5, 1946 OFFICE DRYING OVEN William Bald, Lowell, Ind., assignor to American Can Company, New York, N. Y., a corporation of New Jersey Application March 31, 1942, Serial No. 431,074

.6 Claims.

The present invention relates to drying ovens and has particular reference to drying orbaking lithographed or lacquered sheets of metal. This is an improvement over United States Patent 1,591,683, issued July 6,1926, to C. G. Preis.

An object of the invention is the provision of a sheet metal drying oven wherein a uniform heat distribution with a uniform temperature is maintained throughout the entire length of the oven. 1

Another object is the provision of such an oven wherein its various component parts are ar- Figure 1 is a top plan view of the entrance end of an oven embodying the instant invention, with parts broken away;

Figure 2 is a side elevation of the portion of the apparatus shown in Fig. 1, with. the part broken away; 1 i

Fig. 3 is a top plan view of the discharge end of the oven shown in Fig. 1 and continuing from the left in that figure, Figs. 1 and 3 when joined showing the complete oven with parts broken away;

Fig. 4 is a side elevation of the portion of the oven shown in Fig. 3, with parts broken away, and showing sheets in place in the oven;

Fig. 5 is a top plan view of a sheet feedingmechanism constituting a part of the oven apparatus located at the entrance end of the apparatus at the right in Fig. 1, and also such devices at the discharge end of the oven as viewed on a smaller scale at the left in Fig. 3, with parts broken away;

Fig. 6 is a side elevation of the mechanism illustrated in Fig. 5, also with parts similarly broken away, and showing sheets in place in the mechanism;

Fig. 7 is an enlarged side elevation of the upper portion of the oven near its entrance end as viewed in- Fig. with parts broken away;

Fig. 8 is a top plan view of the portion of the apparatus shown in Fig. '7, with parts broken away;

Fig. 9 is a side elevation showing a continuation of the apparatus shown in Fig. 8, the parts being stantially along the line 20- 20 in Fig. 2, and

located at the upper left in that figure, with parts broken away; I

Fig. 10 is a transverse sectional view taken substantially along the line Ill-40 in Fig. 9;

Fig. 11 is an enlarged side elevation of the entrance end of the machine, with parts broken away;

Fig. 12 is a fragmentary elevational viewtaken substantially along a plane indicated by the line i2-l2 in Fig. 8, and showing an explosion door;

Fig. 13 is a sectional view taken substantially along the line l3-l3 in Fig. 12;

Fig. 14 is an enlarged fragmentary sectional detail showing the hinge construction of the explosion door in Fig. 13;

Fig. 15 is an enlarged sectional view taken substantially along the line l5-| 5 in Fig. 2, and showing a sheet in place in the oven;

Fig. 16 is a sectional detail taken substantially along the line Iii-l6 in Fig. 15, with parts broken away;

Fig. 1'7 is an enlarged sectional view taken substantially along the line l'lil in Fig. 16, with parts broken away;

Fig. 18 is an enlarged side elevation of a cooling section of the oven apparatus shown at the left in Fig. 4, with parts broken away;

Fig. 19 is a transverse section taken substantially along the line l9-l9 in Fig. 18, with parts 1 broken away;

Fig. 20 is an enlarged sectional view taken subshowing a sheet in place in the oven;

Fig. 21 is a sectional view taken substantially along the broken line- 2 l2l in Fig. '7, and showing a sheet in place in the oven;

Fig. 22 is an elevational detail as viewed along a plane indicated by the line 22-22 in Fig. 21;

Fig. 23 is an enlarged perspectiveview of the oven frame and shell construction, with parts broken away; v

Fig. 24 is an enlarged elevational detail of parts located at the discharge end of the oven as viewed in Fig. 4, with parts broken away.

Fig. 25 is an enlarged fragmentary sectional detail taken transversely of the oven in the center and is a. section similar to that shown in Fig. 17 and indicating certain oven connections with the machine frame;

Fig. 26 is an enlarged sectional detail showin the upper rail secured to the frame at the middle; and a Fig. 2''! is an enlarged sectional detail showing the lower rail secured to the frame at the middle.

As a preferred embodiment of the instant in- I vention the drawings illustrate a drying oven of the character disclosed in the above mentioned Preis patent and one in which metallic lithographed sheets A standing on edge are moved slowly through a heated chamber. B so that the lithographing on the sheets will become baked thereon. After this baking operation the sheets are cooled to permit handling for subsequent operations.

The heating chamber B is divided into two connecting compartments, a front compartment through which the sheets first pass and a rear compartment into which the sheets pass from the front compartment. These compartments are enclosed by an arch shaped chamber casing 3| (Figs. 1, 2, 3, 4 and which in some installations may be approximately eighty five feet long and which is made from a plurality of sheet metal transverse sections 32 bolted together. Each section comprises a sheet metal wall 33 secured to a pair of end angle irons 34 and a pair I of bottom channel irons 35. The end angle irons of adjacent oven sections abut as shown in Fig. 23 and are tied together by bolts 36. A gasket 31 interposed between the angle irons provides a leakproof joint.

The two outer oven sections 32 and the middle section are formed with transverse walls 38 each having a rectangular opening 39 therein just large enough to permit the passage of the sheets A therethrough. These walls on the end sections block oil the ends of the even, while the middle wall separates the front and rear compartments.

In such a long oven considerable elongation takes place due to expansion of the sections when the oven is operating under high temperature conditions. To compensate for this elongation the bottom channel iron of each section 32 of the oven is formed with a depending shoe 4| (Fig. 23) which rests on an inverted channel rail 42 extending longitudinally of the oven for its entire length. The shoe of the middle section of the oven is bolted to the rail while the other shoes are free to slide on the rail. Thus the middle section remains stationary while the other oven sections are free to expand in opposite directions from the middle section and thus are prevented from buckling. The upright sides of the channel rails 42 keep the oven sections in line and prevent lateral displacement.

The channel rails 42 are bolted to longitudinal and horizontal angle iron supports 44. These supports are secured to transverse leg sections 45 that rest upon a suitable foundation 46. The supports and the leg sections constitute the main frame of the apparatus.

The exterior surface of the oven sections 32 are covered preferably with heat insulating material 5! and this is held in place by an outer sheet metal shell 52. The lower edges of the shell extend down adjacent the angle iron supports 44 and are bent under and upwardly around the lower edges of the angle irons to secure the shell in place.

The interior of the ven casing 3| is formed with heating channels or spaces BI, 62 (Fig. 15) for circulating a heating medium, such as heated air, through the oven chamber and adjacent the sheets A passing therethrough. These heating channels are disposed adjacent the inside surface of the oven casing and are set off by a sheet metal wall section or shell 63 located in spaced .and parallel relation. to the oven casing. This wall section or shell is secured in place by spacer blocks 64 which are bolted to the wall sectionand to the o'vencasing.

Each oven section 32 is formed with one of thesewalls 63 so that the channels 3!,62 are continuous throughout the two heating compartments of the chamber. The end and middle walls 38 of the corresponding oven sections '32 block oif the outer and inner endsof the channels so that each channel is confined to its corresponding compartment. A radial division plate 66 located near the arched top of the oven sections and interposed between the wall section 63 and the oven. casing 3| separates the channels BI, 62.

Heated air introduced into the channel 6|, in a manner to be hereinafter explained, passes down in the direction of the arrows and through the heating space toward the bottom of the oven, as viewed in Fig. 15. Adjacent the bottom of the even a curved deflector 58 having a plurality of vertical transverse vanes 68 (Fig. 16) directs the air upwardly through an opening Ill (see also Fig. 17) which is restricted or controlled by a hinged damper 1!. The deflected air passes into the chamber B and travels upwardly along the inner curved wall section or shell 63 and adjacent the sheets moving through the chamber.

As the hot air crosses the arched portion of the wall section and travels downwardly along the opposite side of the chamber it tends to spread out and come into direct contact with the sheets being dried. This is prevented during the first part of the drying operation in order to avoid burning of the still wet lithographing as by a longitudinal deflector plate 14 (Fig. 15) which extends alongthis side of the chamber for a distance of approximately two thirds of the length of the front heating compartment. The deflector plate is supported on brackets 15 which are secured to the inner surface of the wall section 63.

After leaving the deflector plate 14, the heated air passes downwardly and along the bottom of the chamber toward the curved deflector '68. The deflector is formed with a downwardly curved member 11 that directs the air through an opening 18 which is regulated by a slide damper I9 (Fig. 1'7) carried in suitable slideways formed in the bottom section of the wall or shell 63. Air

entering this opening passes into the heating channel 62 through which it moves upwardly along the oven casing toward the channel dividing plate 66. At the dividing plate the air is exhausted, reheated and again circulated through the oven in a manner which will be hereinafter explained.

Regulation of the dampers TI, I9 disposed adjacent the curved deflectors 68, I1 is effected from outside the oven (see Fig. 1'7). For this purpose the hinged damper II is pivotally connected to the inner end of a horizontally disposed link 8|. The link extends across the deflector 88 and terminates in an adjusting bracket 82 (see also Fig. 16) secured to the outer surface of the oven casing 3|.

The terminal end of the link is formed preferably with a universal joint ball 83 which is retained in any suitable manner in a threaded adjusting nut 84. The nut is located in a threaded bore 85 formed in the bracket 82. Thus by rotating the nut with a suitable tool such as a wrench;

the link-8i is moved horizontally and the hinged damper H .is hinged into a desired position to regulate the size of the opening restricted thereasoass-z by. There is one of these damper regulating devices for each section 32 of the oven.

the bracket 02.

The heating medium for the oven chamber B is supplied by way of a sheet metal duct IOI (Figs 1, 2, 3, 4 and '7) which is mounted on top of the trifugal fan blower I02. In order to prevent undue vibration of the oven, the blower is carried on a separate auxiliary frame I03 that straddles the oven. The fan is mounted on'a shaft I05 which is journaled in bearings I06 formed in a bracket I01 bolted to the auxiliary frame. inner bearing I06 which is located nearest the duct is cooled by water circulated through pipes III connecting with a supply tank II2 mounted on the auxiliary frame. Y

The fan of the blower I02 is rotated at high speed by way of a belt II4 that operates over a pulley I I5 mounted on the fan shaft I05 and over a pulley I I6 (Figs. 1 and 8) which is carried on a driving shaft of an electric motor I I1. The motor is supported on the auxiliary frame I 03 and is actuated from any suitable source of electric energy.

The blower I02 is connected to an intake hood I2I (Figs. 1, 3, 12, 13 and 21) which is mounted on the chamber casing 3I and which communicates with the exhaust heating passageway 62 of the heating chamber. It is by way of thiscon- The.

curved baflle plates m which extend down into the inlet channel and which deflect the heated. air toward the inner end of the heating compartment which they serve. There is one of these inlet hoods for each heating compartment another I are of identical construction.

Between the burner BI and the main blower I02 avent stack I41 (Figs. 7 and 8). is provided f in the duct IOI for venting the duct of unburned gas when the burner is first ignited. This vent stack is formed with a damper I48 which may be actuated as desired to provide the required venting action. Explosion doors I5I, I52 (see also Figs. 12, 13 and 14) are also provided in the inlet hood I M and in the intake hood I2I to prevent damage to the oven in case ofan explosion of unburned gas which may collect in the duct These doors fit tightly within openings formed in the hoods but are easily pushed out by an extra pressure created within the hoods. The door in the intake hood preferably is mounted on hinges so that it will swing open readily.

At the entrance end of the oven at the right in Figs. 1, 2 and 11) the'chamber casing ,3I is formed with a canopy I55 which curves downwardly over the freshly lithographed sheetsA to be dried as they enter the heating chamber. This canopy section is bolted to the first oven section 32 and rests unattached on top of a pair'of brackets I50. These project outwardly from a sub-frame I56 so that the canopy may slide on the brackets with the expansion and contraction of the oven sections. The sub-frame is mounted on a suitable foundation which may be the same foundation on which the. main frame rests. I Y

The canopy section I55 collects the fumes from the freshly coated sheets and these fumes are drawn off by an exhaust duct I 51 which extends up from the top of the canopy. An exhaust fan I58 is located within the duct and is nection that the heated air after circulating through the oven is drawn up by the blower I02 into the heating duct IOI for reheating and for re-circulation through the oven. Fresh air is drawn into the blower by way of a register I23 (see Fig. 22) which is provided in the side of the oven casing 3I. This register includes a pair of register openings I24, the size of which may be regulated manually by a rotatable cover plate I25.

Air blown into the duct IN is heated prior to being circulated through the oven, by a gas flame burner I3I (Figs. 7 and 8) which is located in one side of the duct. The burner is furnished with gas from a pipe line I32 which leads from a suitable supply cf gas. The burner is ignited by means of a pilot light enclosed in the burner and.

with the burner and which is actuated by an electric motor I36. The blower and the motor are supported on a bracket I31 that extends out from the auxiliary frame I03.

The heated air passing through the duct IN is directed into the oven chamber inlet channel 6I by way of an inlet hood I4I (see also Figs. 9 and 10). This inlet hood is secured to the tops of two of the oven sections 32, as shown in Figs. 2 and 4-, and communicates with openings I42 (Fig. 20) therein that lead into the inlet channel 6I. The bottom of this inlet hood is formed with an open grating I43 and with a plurality of operated by an electric motor I59. The motor. is disposed outside the duct on a bracket I60 of the auxiliary frame 103 and draws the fumes off to any suitable place of deposit.

The sheets to be dried are received from any suitable source of supply, such as a printing or coating machine and are conveyed through the oven on an endless chain conveyor I15 (Figs. 4, 5, 6 and 15). The conveyor includes a pair of spaced and parallel chains I16 having closely spaced wickets or flights I11 secured thereto and extending along its entire length. These chains operate over two pairs of driving sprockets I8I, I82. One pair is located at each end of the oven. There are two pairs of idler sprockets I83, I84, one pair being disposed adjacent each pair of the driving sprockets.

The driving sprockets I8I are located at the feed-in or entrance end of the oven and are mounted 'on a driving shaft I86 journaled in bearings I81 formed in a movable'bracket I88 secured by bolts I89 to the top of the sub-frames I56. The driving shaft is rotated by a driving sprocket I92 mounted on the shaft and connected by an endless chain I93 to a speed reduction unit I94. The reduction unit is operated by a chain I95 which is actuated preferably by a main electric motor (not shown). This-motor preferably is used to drive the coating or printing machine that applies the lithographing to the sheets A.

The idler sprockets I83 are located'at the en- I trance end of the oven and are disposed between the upper and lower runs of the conveyor chains .stretch at both ends of the oven.

to maintain them in spaced relation. These idler sprockets are mounted on a cross-shaft 20I- carried in bearings 202 formed in the movable bracket I88.

The driving sprockets I82 at the opposite or discharge end of the oven are mounted on an auxiliary driving shaft 205 journaled in bearings 206 formed in *a sub-frame 201 which rests on the oven foundation. ing shaft is driven by a sprocket 208 which is mounted on one end of the shaft. The sprocket is rotated by an endless chain 2 that operates over a driving sprocket 212 mounted on the rotor shaft of a synchronous electric motor 2l3. Motor H3 is actuated in synchronism with the main motor hereinbefore mentioned.

The idler sprockets I84 are located at the discharge end of the oven and are mounted on a cross-shaft 2I5 carried in stationary brackets 2I6 secured to a pair of cross-rails 2I'I which constitute a part of the sub-frame 201. These idler sprockets I84 operate against the lower runs of the conveyor chains and maintain these runs in spaced relation to the upper runs at this end of the oven.

In such long chains considerable stretch often results and provision is made for taking up this Any permanent stretch is taken up at the entrance end of the oven by shifting the movable bracket I88. For this purpose the bolts I89 which hold the bracket extend through elongated slots 22I in the bracket.

The bracket I88 is shifted manually by turnin a handle 222 which is secured to a cross-shaft 223 carried in bearings formed in a pair of housings 224 secured to the sub-frame I 56. Within the housings the cross-shaft carries a pair of worms which mesh with worm-wheels carried on a pair of rotatable rams 225. The rams abut against the inner edge of the movable bracket I88 and when the bolts I89 are loosened and the handle .222 is rotated, the rams are rotated against the bracket and slide it along the top of the sub-frame. This movement of the bracket carries driving sprockets I8I and idler sprockets I83 along with it and thus draws the conveyor chains taut.

At the discharge end of the oven, the conveyor chains are maintained in a taut condition by a pair of weight arms 23I (Figs. 5 and 6) secured to the outer ends of a cross-shaft 232. The cross-shaft carries a pair of tightener sprockets 234 that rotate against the lower run of the conveyor chains. The cross-shaft is carried in a pair of swing arms 235 loosely mounted on the auxiliary drive shaft 205. Weights 235 secured to the weight arms press the tightener sprockets against the conveyor chains and thus maintain them in a taut condition.

Between the driving sprockets IBI, I82, the upper and lower runs of the conveyor chains are supported and are maintained in a horizontal spaced relation by longitudinal conveyor rails 2. At spaced intervals, the chains carry outwardly projecting rollers 242 that ride on these conveyor rails (see also Fig. The rails extend the full length of the conveyor and are carried loosely in grooved or slotted brackets 243 bolted to the cross-rails of the frame and to the sub-frames of the oven and also on the'bottom section of the channel wall 63 which constitutes the bottom of the oven chamber This auxiliary driv-.

(Fig. 15). The rails at their intermediate portions ar bolted to the slotted brackets.

Hence the rails 24] are free to expand in 0D- posite directions and thereby buckling of the rails or creeping in the direction of travel of the conveyor is prevented. The looseconnection between the brackets and the rails permits of this expansion while maintaining the rails in place against lateral displacement.

The conveyor travels through the oven very slowly and carries the sheets in an upright position or on edge as shown in Fig. 6. Upon completing this travel through the oven, the conweyor continues on through a cooling unit 250 (Figs. 3 and 4) in which the high temperature of the baked sheets is reduced sufliciently to permit handling.

The cooling unit 250 includes a rectangular sheet metal housing 25I (Figs. 18 and. 19) having an opening 252 in each end large enough to permit the passage of a sheet A therethrough. The housing extends below the upper run of the conveyor I15 and is supported on an auxiliary frame 253 which is a continuation of the main frame 45. The housing located below the upper run of the conveyor, carries three cooling fans 255 that are mounted on the rotor shafts of three separate electric motors 256 located directly below the conveyor. These motors are actuated from any suitable source of electric energy. Adjacent .the motors the housing is conected to a fresh air inlet duct 25] that extends upwardly alongside the housing.

Hence when the motors 250 are in operation, the fans 255 draw in fresh airfrom the duct 25'! and blow it upwardly between the sheets A passing through the housing. It is this air that cools the sheets. Air circulated between the sheets is caught by a plurality of exhaust fans 26I located directly above the path of travel of the sheets. There are three of these fans and they are rotated by three separate electric motors 282 that are"'actuated from any suitable source of electric energy. These fans draw the air up between the sheets and blow it out through an exhaust stack 253 formed on top of the housing 25L In order to prevent undue chilling of the sheets A as they pass through the cooling section, provision is made to recirculate apart of the air that is forced up between the hot sheets. For this purpose there is a by-pass connection 265 between the air inlet duct 25'! and the cooling weather conditions when the fresh air entering the duct 251 may be too cold, it may be modulated or tempered by adjustment of the damper so that a portion of the air blown on the sheets and partially heated thereby may be directed through the by-pass and thus mix with the fresh air to reduce its temperature to within desired limits.

After leaving the cooling section 25I the cooled sheets A are ready for discharge from the conveyor. This may be performed manually or automatically as desired. Usually the sheets are removed manually and are arranged in stacks on hand trucks for subsequent operations. This removal of the sheets is facilitated by a pair of inclined, stationary lifter rails 2' of the conveyor so that they may be readilygrasped by the operator.

The lifter rails 21! preferably are straight for their major xtent and are formed with a curved discharge end 212. There are two of these lifter rails located one on each side of the conveyor H5. The straight portion of the rails are formed with depending lugs 213 having elongated slots 214 through which clamping bolts 215 extend into stationary brackets 216 mounted on the cross rails 2| 1 of the auxiliary sub-frame 201. curved ends of the lifter rails are formed with lugs 21! which are pivotally connected toslotted links 218 bolted to brackets 219. Brackets 219 are secured to the driving shaft bearings 206 of the auxiliary sub-frame. With this construction of support members for the lifter rail adjustment may be made to any desired inclination and height to facilitate removal of l the sheets.

It is thought that the invention and many of its attendant advantages will be understood from the foregoing description, and it will be apparent that various changes may be made in the form, construction and arrangement of the parts without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the form hereinbefore described being merely a preferred embodiment thereof.

I claim:

1. A drying oven for lithographed metal sheets comprising in combination, a frame, an elongated casing enclosing a drying chamber mounted on said frame, a conveyor operating through said chamber for carrying lithographed metal sheets to be dried, longitudinally disposed spaced rails for supporting said conveyor, a transverse wall conveyor, a runway on said frame for loosely supporting said casing sections so that the same are free to expand under hightemperaturos in the chamber, and means forsecuring said rails at their intermediate portions to said frame to permit longitudinal expansion of the rails in opposite directions from their secured portions under for supporting said conveyor, means for circulating heated air through the chamber for drying the sheets moving therethrough, means disposed adjacent the discharge end of said conveyor for elevating the sheets to a desired position in readiness for discharge from the conveyor, and means for securing the intermediate portions of said rails to said frame to permit longitudinal expansion of the rails in opposite directions from their said secured portions under varying temperature within said casing and dividing said chamber into separate compartments, said wall having an opening therein for the passage of the conveyor and the sheets therethrough, a plurality of heating devices for individually heating the chamber compartments to maintain predetermined temperatures therein, and means for securing said rails at their intermediate portions to said frame to permit longitudinal expansion of the rails in opposite directions from their secured intermediate portions under varying temperature conditions.

2. A drying oven for lithographed metal sheets, comprising in combination, a casing enclosing a drying chamber and having a supporting frame, means for heating said chamber, a conveyor operating through said chamber for carrying lithographed metal sheets to be dried, and continuous rails extending through said chamber for guiding the conveyor, each rail being secured at its intermediate portion to said frame and being free throughout the remainder of its length, to allow unrestricted longitudinal expansion under high temperatures thereby preventing creeping of the rails in the direction of travel of the conveyor.

3. A drying oven for lithographed metal sheets comprising in combination: a frame. a plurality of oven sections secured together to provide a casing enclosing an elongated drying chamber mounted on said frame, means for heating said chamber, an endless conveyor operating through conditions.

5. A drying oven for lithographed metal sheets comprising in combination, a frame, an outer casing enclosing a drying chamber supported by said frame, an endless conveyor operating through said chamber and including spaced wickets for carrying lithographed sheets on edge to be dried in said chamber, longitudinally disposed spaced rails in said chamber for supporting said conveyor, means for circulating heated air through said chamber for drying the sheets moving therethrough, a pair of adjustable inclined rails disposed adjacent said conveyor at its discharge end for raising the sheets above said wickets as they ride up on said rails so that the sheets may be readily grasped for removal from the conveyor, and means for securing said longitudinally spaced rails at intermediate portions thereof to said frame to permit longitudinal expansion of the rails in opposite directions from their said secured portions to compensate for varying temperature conditions.

6. A drying oven for lithographing metal sheets, comprising a sectional casing enclosing a drying chamber, a conveyor for carrying the sheets in an upright position through said chamber, a heating wall in each of the sections of said casing and spaced from the casing to constitute therewith an 'air passage extending substantially around the outer part of said chamber, independent means at opposite ends of said chamber for directing heated air inwardly through said passage, means for circulating saidheated air transverseiy to the path of travel of said sheets through said chamber and immediately around the moving sheets on said conveyor, a pair of separate spaced movable dampers disposed in one of said casing sections respectively at opposite ends of the transverse path of travel of the heated air within the chamber for controlling the flow of heated air through said passage, and means-disposed in said casing and operable from exterior thereof for independently and selectively shifting said dampers to a desired position, whereby to vary the volume of the heated air admitted to said chamber.

WILLIAM BALD. 

